I am trying to get some ideas of what works well for people when the mercury dips below freezing. Tell me what layers you have and what you like and what you would change about the system. I am still fiddling with my system but here is what I have for this winter so far.

Sleeping Pad - 3/8" CCF Gossamer Gear Thinlight on top of large Neoair

Base - light merino base layer

Insulation Clothing - none

VBL - AMK heetsheets bivy

Sleeping Bag - 0 degree down bag (undecided)

Bivy - Montbell Breeze Dry-Tec bag cover

So far the pad system seems to be working out well and is reasonably light and warm. I love merino and think it will always be my base layer. I am still trying to figure out the best way to use a VBL. I am considering VB clothing in lieu of a liner so I can wear insulative layers over it. I am undecided on the sleeping bag. Currently I am just using in my WM Highlite bag. That will change when it gets colder. I am also torn on the bivy. I feel it adds warmth to the sleeping system but seems perhaps overkill when used in a shelter. Is it a good idea to use a bivy just to protect from frost from outside?

I think a bivy is overkill in a decent shelter, especially in a double wall tent. I also think it would sometimes contribute to moisture build-up in the bag, and that would make VBL use more important. I plan to experiment some with bag VBL this winter.

It was slightly drafty due to the winds and weight gain. I'm sure it would have been perfectly comfortable if I hadn't gained a lot of weight since buying the quilt. I wasn't cold though, just annoyed by the drafts.

I also had a blue foam sit pad that I put under my hips...not because I needed extra warmth, but because I wanted to try doing something useful with it.

My experience has been pretty equivalent to the above numbers, including plenty of time in the -20F to 10F range.

A Neo plus a Thinlight is something like R-3 to 3.5, ie half of the thermoneutral value for 0F. In my experience, I've found that R-8 is necessary around -20F to 10F (R6 acceptable at 10+), & a Downmat 9 works great (R-8). Have also used a standard Thermarest w/a ridge rest quite happily (~R-6). If the pad isn't warm enough, the rest of your system (including you) has to work harder to stay warm. Not saying that it isn't possible to use a lower R-value pad in such temps, but that it is unusual for that lower R-value to be sufficient for most people.

Also note that when sleeping bags are rated at a given temp, they assume an adequate amount of insulation underneath them, as above. In other words, if you have a negative 20F bag on a thinlight in +20F weather, you'll probably be cold.

Just to clarify from the OP, that no insulating clothing bit was specifically for the sleep system, right, but you would have insulating garments in your pack?

One other note, on the large Neo... the wider pad w/a narrower bag can result in some additional airflow/heat loss. I talked to someone at Cascade about this & they agreed... better in cold to have the sleeping bag pretty much covering the pad.

Interesting, Brad. I guess that means one sleeps on frozen ground or rock, for those number?

My experience is that when camping on top of snow, an R Value of between 4 and 5 is sufficient, reason being that the temperature of snow is at the 0 C mark. Hence I don't bring a Downmat or similar heavy equipment when I will be sleeping on top of snow, a Multimat Adventure and a Artiach Skin-Micro Lite are more than sufficient for that.

I think those ratings are useful, but also a wee bit cautious (they probably don't want them going the other way!)

I've used a neo air several times at or slightly below the freezing mark w/ no discomfort (I sleep a little warm so maybe that's some of it), according to the EN chart I should have two Neo's. I've also got by into the mid 20's w/ a neo and a 1/8" thinlight- again contrary to the EN ratings.

I think the ratings are useful, but some (probably not all) might find them a little on the cautious side- which is a much better scenario than the opposite :)